Molecular Dynamics Simulations of Cellulose and Dialcohol Cellulose under Dry and Moist Conditions.

The development of wood-based thermoplastic polymers that can replace synthetic plastics is of high environmental importance, and previous studies have indicated that cellulose-rich fiber containing dialcohol cellulose (ring-opened cellulose) is a very promising candidate material. In this study, molecular dynamics simulations, complemented with experiments, were used to investigate how and why the degree of ring opening influences the properties of dialcohol cellulose, and how temperature and presence of water affect the material properties. Mechanical tensile properties, diffusion/mobility-related properties, densities, glass-transition temperatures, potential energies, hydrogen bonds, and free volumes were simulated for amorphous cellulosic materials with 0-100% ring opening, at ambient and high (150 °C) temperatures, with and without water. The simulations showed that the impact of ring openings, with respect to providing molecular mobility, was higher at high temperatures. This was also observed experimentally. Hence, the ring opening had the strongest beneficial effect on "processability" (reduced stiffness and strength) above the glass-transition temperature and in wet conditions. It also had the effect of lowering the glass-transition temperature. The results here showed that molecular dynamics is a valuable tool in the development of wood-based materials with optimal thermoplastic properties.

Elucidating the fine-scale structural morphology of nanocellulose by nano infrared spectroscopy.

Nanoscale infrared (IR) spectroscopy and microscopy, enabling the acquisition of IR spectra and images with a lateral resolution of 20 nm, is employed to chemically characterize individual cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs) to elucidate if the CNCs and CNFs consist of alternating crystalline and amorphous domains along the CNF/CNC. The high lateral resolution enables studies of the nanoscale morphology at different domains of the CNFs/CNCs: flat segments, kinks, twisted areas, and end points. The types of nanocellulose investigated are CNFs from tunicate, CNCs from cotton, and anionic and cationic wood-derived CNFs. All nano-FTIR spectra acquired from the different samples and different domains of the individual nanocellulose particles resemble a spectrum of crystalline cellulose, suggesting that the non-crystalline cellulose signal observed in macroscopic measurements of nanocellulose most likely originate from cellulose chains present at the surface of the nanocellulose particles.

Green Ambient-Dried Aerogels with a Facile pH-Tunable Surface Charge for Adsorption of Cationic and Anionic Contaminants with High Selectivity.

The fabrication of reusable, sustainable adsorbents from low-cost, renewable resources via energy efficient methods is challenging. This paper presents wet-stable, carboxymethylated cellulose nanofibril (CNF) and amyloid nanofibril (ANF) based aerogel-like adsorbents prepared through efficient and green processes for the removal of metal ions and dyes from water. The aerogels exhibit tunable densities (18-28 kg m-3), wet resilience, and an interconnected porous structure (99% porosity), with a pH controllable surface charge for adsorption of both cationic (methylene blue and Pb(II)) and anionic (brilliant blue, congo red, and Cr(VI)) model contaminants. The Langmuir saturation adsorption capacity of the aerogel was calculated to be 68, 79, and 42 mg g-1 for brilliant blue, Pb(II), and Cr(VI), respectively. Adsorption kinetic studies for the adsorption of brilliant blue as a model contaminant demonstrated that a pseudo-second-order model best fitted the experimental data and that an intraparticle diffusion model suggests that there are three adsorption stages in the adsorption of brilliant blue on the aerogel. Following three cycles of adsorption and regeneration, the aerogels maintained nearly 97 and 96% of their adsorption capacity for methylene blue and Pb(II) as cationic contaminants and 89 and 80% for brilliant blue and Cr(VI) as anionic contaminants. Moreover, the aerogels showed remarkable selectivity for Pb(II) in the presence of calcium and magnesium as background ions, with a selectivity coefficient more than 2 orders of magnitude higher than calcium and magnesium. Overall, the energy-efficient and sustainable fabrication procedure, along with good structural stability, reusability, and selectivity, makes these aerogels very promising for water purification applications.

Microscopic Insight into the Structure-Processing-Property Relationships of Core-Shell Structured Dialcohol Cellulose Nanoparticles.

In the quest to develop sustainable and environmentally friendly materials, cellulose is a promising alternative to synthetic polymers. However, native cellulose, in contrast to many synthetic polymers, cannot be melt-processed with traditional techniques because, upon heating, it degrades before it melts. One way to improve the thermoplasticity of cellulose, in the form of cellulose fibers, is through chemical modification, for example, to dialcohol cellulose fibers. To better understand the importance of molecular interactions during melt processing of such modified fibers, we undertook a molecular dynamics study of dialcohol cellulose nanocrystals with different degrees of modification. We investigated the structure of the nanocrystals as well as their interactions with a neighboring nanocrystal during mechanical shearing, Our simulations showed that the stress, interfacial stiffness, hydrogen-bond network, and cellulose conformations during shearing are highly dependent on the degree of modification, water layers between the crystals, and temperature. The melt processing of dialcohol cellulose with different degrees of modification and/or water content in the samples was investigated experimentally by fiber extrusion with water used as a plasticizer. The melt processing was easier when increasing the degree of modification and/or water content in the samples, which was in agreement with the conclusions derived from the molecular modeling. The measured friction between the two crystals after the modification of native cellulose to dialcohol cellulose, in some cases, halved (compared to native cellulose) and is also reduced with increasing temperature. Our results demonstrate that molecular modeling of modified nanocellulose fibers can provide fundamental information on the structure-property relationships of these materials and thus is valuable for the development of new cellulose-based biomaterials.

The structure of cellulose nanofibril networks at low concentrations and their stabilizing action on colloidal particles.

The structure and dynamics of networks formed by rod-shaped particles can be indirectly investigated by measuring the diffusion of spherical tracer particles. This method was used to characterize cellulose nanofibril (CNF) networks in both dispersed and arrested states, the results of which were compared with coarse-grained Brownian dynamics simulations. At a CNF concentration of 0.2 wt% a transition was observed where, below this concentration tracer diffusion is governed by the increasing macroscopic viscosity of the dispersion. Above 0.2 wt%, the diffusion of small particles (20-40 nm) remains viscosity controlled, while particles (100-500 nm) become trapped in the CNF network. Sedimentation of silica microparticles (1-5 µm) in CNF dispersions was also determined, showing that sedimentation of larger particles is significantly affected by the presence of CNF. At concentrations of 0.2 wt%, the sedimentation velocity of 5 µm particles was reduced by 99 % compared to pure water.

Water-resistant hybrid cellulose nanofibril films prepared by charge reversal on gibbsite nanoclays.

A novel method is reported for the preparation of a hybrid gibbsite-cellulose nanofibril (CNF) nanocomposite film with improved wet and dry mechanical properties and barrier properties. A gibbsite and cationic CNF dispersion was dewatered at pH 7 to prepare well-ordered films. Thereafter, the charge on gibbsite was reversed by dipping the film in pH 12 water to induce an ionic interaction between CNFs and gibbsite, enhancing the film properties; modulus improved from 9 GPa to 12 GPa, with a maintained strain-at-break of 6 % and tensile strength of 190 MPa. Additionally, the charge-reversed film swelled a factor of 24 less than a film without any gibbsite. At 23 °C and 80 % RH, the oxygen barrier properties were improved by a factor of 28, to a value of 18 ml·µm·m-2·kPa-1·24 h-1 and the water vapour barrier properties were improved by a factor of 12, to a value of 105 g·µm·m-2·kPa-1·24 h-1.

Shaping 90 wt% NanoMOFs into Robust Multifunctional Aerogels Using Tailored Bio-Based Nanofibrils.

Metal-organic frameworks (MOFs) are hybrid porous crystalline networks with tunable chemical and structural properties. However, their excellent potential is limited in practical applications by their hard-to-shape powder form, making it challenging to assemble MOFs into macroscopic composites with mechanical integrity. While a binder matrix enables hybrid materials, such materials have a limited MOF content and thus limited functionality. To overcome this challenge, nanoMOFs are combined with tailored same-charge high-aspect-ratio cellulose nanofibrils (CNFs) to manufacture robust, wet-stable, and multifunctional MOF-based aerogels with 90 wt% nanoMOF loading. The porous aerogel architectures show excellent potential for practical applications such as efficient water purification, CO2 and CH4 gas adsorption and separation, and fire-safe insulation. Moreover, a one-step carbonization process enables these aerogels as effective structural energy-storage electrodes. This work exhibits the unique ability of high-aspect-ratio CNFs to bind large amounts of nanoMOFs in structured materials with outstanding mechanical integrity-a quality that is preserved even after carbonization. The demonstrated process is simple and fully discloses the intrinsic potential of the nanoMOFs, resulting in synergetic properties not found in the components alone, thus paving the way for MOFs in macroscopic multifunctional composites.

Surface tailoring of cellulose aerogel-like structures with ultrathin coatings using molecular layer-by-layer assembly.

Cellulose nanofibril-based aerogels have promising applicability in various fields; however, developing an efficient technique to functionalize and tune their surface properties is challenging. In this study, physically and covalently crosslinked cellulose nanofibril-based aerogel-like structures were prepared and modified by a molecular layer-by-layer (m-LBL) deposition method. Following three m-LBL depositions, an ultrathin polyamide layer was formed throughout the aerogel and its structure and chemical composition was studied in detail. Analysis of model cellulose surfaces showed that the thickness of the deposited layer after three m-LBLs was approximately 1 nm. Although the deposited layer was extremely thin, it led to a 2.6-fold increase in the wet specific modulus, improved the acid-base resistance, and changed the aerogels from hydrophilic to hydrophobic making them suitable materials for oil absorption with the absorption capacity of 16-36 g/g. Thus, demonstrating m-LBL assembly is a powerful technique for tailoring surface properties and functionality of cellulose substrates.

Advanced Characterization of Self-Fibrillating Cellulose Fibers and Their Use in Tunable Filters.

Thorough characterization and fundamental understanding of cellulose fibers can help us develop new, sustainable material streams and advanced functional materials. As an emerging nanomaterial, cellulose nanofibrils (CNFs) have high specific surface area and good mechanical properties; however, handling and processing challenges have limited their widespread use. This work reports an in-depth characterization of self-fibrillating cellulose fibers (SFFs) and their use in smart, responsive filters capable of regulating flow and retaining nanoscale particles. By combining direct and indirect characterization methods with polyelectrolyte swelling theories, it was shown that introduction of charges and decreased supramolecular order in the fiber wall were responsible for the exceptional swelling and nanofibrillation of SFFs. Different microscopy techniques were used to visualize the swelling of SFFs before, during, and after nanofibrillation. Through filtration and pH adjustment, smart filters prepared via in situ nanofibrillation showed an ability to regulate the flow rate through the filter and a capacity of retaining 95% of 300 nm (diameter) silica nanoparticles. This exceptionally rapid and efficient approach for making smart filters directly addresses the challenges associated with dewatering of CNFs and bridges the gap between science and technology, making the widespread use of CNFs in high-performance materials a not-so-distant reality.

On the interaction between PEDOT:PSS and cellulose: Adsorption mechanisms and controlling factors.

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a conducting polymer frequently used with cellulose, to develop advanced electronic materials. To understand the fundamental interactions between cellulose and PEDOT:PSS, a quartz crystal microbalance with dissipation (QCM-D) was used to study the adsorption of PEDOT:PSS onto model films of cellulose-nanofibrils (CNFs) and regenerated cellulose. The results show that PEDOT:PSS adsorbs spontaneously onto anionically charged cellulose wherein the adsorbed amount can be tuned by altering solution parameters such as pH, ionic strength and counterion to the charges on the CNF. Temperature-dependent QCM-D studies indicate that an entropy gain is the driving force for adsorption, as the adsorbed amount of PEDOT:PSS increased with increasing temperature. Colloidal probe AFM, in accordance with QCM-D results, also showed an increased adhesion between cellulose and PEDOT:PSS at low pH. AFM images show bead-like PEDOT:PSS particles on CNF surfaces, while no such organization was observed on the regenerated cellulose surfaces. This work provides insight into the interaction of PEDOT:PSS/cellulose that will aid in the design of sustainable electronic devices.

Self-Fibrillating Cellulose Fibers: Rapid In Situ Nanofibrillation to Prepare Strong, Transparent, and Gas Barrier Nanopapers.

Cellulose nanofibrils (CNFs) prepared from wood biomass are promising candidates to replace oil-based materials in, for example, packaging applications. However, CNFs' affinity for water combined with their small size leads to very slow and energy-demanding processes for handling and removal of water. To a large extent, this is the major roadblock that prevents a feasible production of dry CNF-based materials on an industrial scale. In this work, self-fibrillating fibers (SFFs) from wood, where the fibrils can be liberated by external stimuli, were prepared via sequential TEMPO and periodate oxidation reactions. Papers made from these modified fibers using conventional laboratory papermaking methods were then in situ nanofibrillated via a modest pH increase. With a dewatering time of less than 10 s for a 3 g/L dispersion, SFFs represent a major improvement over conventional CNF nanopapers that take approximately 6 h to dewater. Moreover, 100 g/m2 nanopapers obtained through in situ fibrillation exhibited comparable, if not superior, properties to those reported for conventionally made CNF films. A tensile strength of 184 MPa, a Young's modulus of 5.2 GPa, a strain at break of 4.6%, 90% optical transmittance, and an oxygen permeability of 0.7 cm3 µm m-2 d-1 kPa-1 at 50% RH were measured for SFF nanopapers. Furthermore, in situ nanofibrillation of the SFFs can also be achieved from already dried papers, facilitating numerous possibilities in terms of logistics and handling for an industrial scale-up and transportation of nanomaterials. Overall, stimuli-induced SFFs indeed enable a rapid production of strong, transparent, gas barrier nanopapers, which likely can be industrially scaled up and eventually compete with the oil-based plastics in the market for packaging materials.

Toward Improved Understanding of the Interactions between Poorly Soluble Drugs and Cellulose Nanofibers.

Cellulose nanofibers (CNFs) have interesting physicochemical and colloidal properties that have been recently exploited in novel drug-delivery systems for tailored release of poorly soluble drugs. The morphology and release kinetics of such drug-delivery systems heavily relied on the drug-CNF interactions; however, in-depth understanding of the interactions was lacking. Herein, the interactions between a poorly soluble model drug molecule, furosemide, and cationic cellulose nanofibers with two different degrees of substitution are studied by sorption experiments, Fourier transform infrared spectroscopy, and molecular dynamics (MD) simulation. Both MD simulations and experimental results confirmed the spontaneous sorption of drug onto CNF. Simulations further showed that adsorption occurred by the flat aryl ring of furosemide. The spontaneous sorption was commensurate with large entropy gains as a result of release of surface-bound water. Association between furosemide molecules furthermore enabled surface precipitation as indicated by both simulations and experiments. Finally, sorption was also found not to be driven by charge neutralization, between positive CNF surface charges and the furosemide negative charge, so that surface area is the single most important parameter determining the amount of sorbed drug. An optimized CNF-furosemide drug-delivery vehicle thus needs to have a maximized specific surface area irrespective of the surface charge with which it is achieved. The findings also provide important insights into the design principles of CNF-based filters suitable for removal of poorly soluble drugs from wastewater.

Chemical modification of cellulose-rich fibres to clarify the influence of the chemical structure on the physical and mechanical properties of cellulose fibres and thereof made sheets.

Despite the different chemical approaches used earlier to increase the ductility of fibre-based materials, it has not been possible to link the chemical modification to their mechanical performance. In this study, cellulose fibres have been modified by periodate oxidation, alone or followed either by borohydride reduction, reductive amination or chlorite oxidation. In addition, TEMPO oxidation, and TEMPO oxidation in combination with periodate oxidation and further reduction with sodium borohydride have also been studied. The objective was to gain understanding of the influence of different functional groups on the mechanical and structural properties of handsheets made from the modified fibres. It was found that the modifications studied improved the tensile strength of the fibres to different extents, but that only periodate oxidation followed by borohydride reduction provided more ductile fibre materials. Changes in density, water-holding capacity and mechanical performance were also quantified and all are dependent on the functional group introduced.

On the origin of sorption hysteresis in cellulosic materials.

Moisture sorption and moisture sorption hysteresis of carbohydrates are phenomena which affect the utilisation of products made thereof. Although extensively studied, there is still no consensus regarding the mechanisms behind sorption hysteresis. Attempts have been made to link the behaviour to molecular properties, in particular to softening properties, and the moisture sorption hysteresis has therefore here been investigated by modifying cellulosic fibres to affect their softening properties. The results show that the moisture sorption hysteresis diminishes with decreasing softening temperature, and was even completely absent at the higher degrees of modification. The moisture sorption characteristics also changed from a type II sorption to a more type III sorption behaviour, a feature more prominent the higher the degree of modification and the higher the temperature. For the highest degree of modification studied the sorption characteristics changed from sorbing less water the higher the temperature to sorbing more water with increasing temperature.

Bacterial adhesion to polyvinylamine-modified nanocellulose films.

Cellulose nanofibril (CNF) materials have been widely studied in recent years and are suggested for a wide range of applications, e.g., medical and hygiene products. One property not very well studied is the interaction between bacteria and these materials and how this can be controlled. The current work studies how bacteria adhere to different CNF materials modified with polyelectrolyte multilayers. The tested materials were TEMPO-oxidized to have different surface charges, periodate-oxidized to vary the water interaction and hot-pressed to alter the surface structure. Then, multilayers were constructed using polyvinylamine (PVAm) and polyacrylic acid. Both the material surface charge and water interaction affect the amount of polymer adsorbed to the surfaces. Increasing the surface charge decreases the adsorption after the first PVAm layer, possibly due to conformational changes. Periodate-oxidized and crosslinked films have low initial polymer adsorptions; the decreased swelling prevents polymer diffusion into the CNF micropore structure. Microscopic analysis after incubating the samples with bacterial suspensions show that only the materials with the lowest surface charge enable bacteria to adhere to the surface because, when adsorbing up to 5 layers PVAm/PAA, the increased anionic surface charge appears to decrease the net surface charge. Both the amounts of PVAm and PAA influence the net surface charge and thus the bacterial adhesion. The structure generated by the hot-pressing of the films also strongly increases the number of bacteria adhering to the surfaces. These results indicate that the bacterial adhesion to CNF materials can be tailored using polyelectrolyte multilayers on different CNF substrates.

Contact-active antibacterial aerogels from cellulose nanofibrils.

The use of cellulose aerogels as antibacterial materials has been investigated by applying a contact-active layer-by-layer modification to the aerogel surface. Studying the adsorption of multilayers of polyvinylamine (PVAm) and polyacrylic acid to aerogels comprising crosslinked cellulose nanofibrils and monitoring the subsequent bacterial adhesion revealed that up to 26mgPVAmgaerogel(-1) was adsorbed without noticeably affecting the aerogel structure. The antibacterial effect was tested by measuring the reduction of viable bacteria in solution when the aerogels were present. The results show that >99.9% of the bacteria adhered to the surface of the aerogels. Microscopy further showed adherence of bacteria to the surfaces of the modified aerogels. These results indicate that it is possible to create materials with three-dimensional cellulose structures that adsorb bacteria with very high efficiency utilizing the high specific surface area of the aerogels in combination with their open structure.

Strong, Water-Durable, and Wet-Resilient Cellulose Nanofibril-Stabilized Foams from Oven Drying.

Porous materials from cellulose nanofibrils (CNFs) have been prepared using Pickering foams from aqueous dispersions. Stable wet foams were first produced using surface-modified CNFs as stabilizing particles. To better maintain the homogeneous pore structure of the foam after drying, the foams were dried in an oven on a liquid-filled porous ceramic frit. The cell structure was studied by scanning electron microscopy and liquid porosimetry, the mechanical properties were studied by compression testing, and the liquid absorption capacity was determined both with liquid porosimetry and by soaking in water. By controlling the charge density of the CNFs, it was possible to prepare dry foams with different densities, the lowest density being 6 kg m(-3), that is, a porosity of 99.6%. For a foam with a density of 200 kg m(-3), the compressive Young's modulus was 50 MPa and the energy absorption to 70% strain was 2.3 MJ m(-3). The use of chemically modified CNFs made it possible to prepare cross-linked foams with water-durable and wet-resilient properties. These foams absorbed liquid up to 34 times their own weight and were able to release this liquid under compression and to reabsorb the same amount when the pressure was released.

Vibrational Sum Frequency Spectroscopy on Polyelectrolyte Multilayers: Effect of Molecular Surface Structure on Macroscopic Wetting Properties.

Adsorption of a single layer of molecules on a surface, or even a reorientation of already present molecules, can significantly affect the surface properties of a material. In this study, vibrational sum frequency spectroscopy (VSFS) has been used to study the change in molecular structure at the solid-air interface following thermal curing of polyelectrolyte multilayers of poly(allylamine hydrochloride) and poly(acrylic acid). Significant changes in the VSF spectra were observed after curing. These changes were accompanied by a distinct increase in the static water contact angle, showing how the properties of the layer-by-layer molecular structure are controlled not just by the polyelectrolyte in the outermost layer but ultimately by the orientation of the chemical constituents in the outermost layers.

Ductile all-cellulose nanocomposite films fabricated from core-shell structured cellulose nanofibrils.

Cellulosic materials have many desirable properties such as high mechanical strength and low oxygen permeability and will be an important component in a sustainable biomaterial-based society, but unfortunately they often lack the ductility and formability offered by petroleum-based materials. This paper describes the fabrication and characterization of nanocomposite films made of core-shell modified cellulose nanofibrils (CNFs) surrounded by a shell of ductile dialcohol cellulose, created by heterogeneous periodate oxidation followed by borohydride reduction of the native cellulose in the external parts of the individual fibrils. The oxidation with periodate selectively produces dialdehyde cellulose, and the process does not increase the charge density of the material. Yet the modified cellulose fibers could easily be homogenized to CNFs. Prior to film fabrication, the CNF was shown by atomic force microscopy to be 0.5-2 µm long and 4-10 nm wide. The films were fabricated by filtration, and besides uniaxial tensile testing at different relative humidities, they were characterized by scanning electron microscopy and oxygen permeability. The strength-at-break at 23 °C and 50% RH was 175 MPa, and the films could, before rupture, be strained, mainly by plastic deformation, to about 15% and 37% at 50% RH and 90% RH, respectively. This moisture plasticization was further utilized to form a demonstrator consisting of a double-curved structure with a nominal strain of 24% over the curvature. At a relative humidity of 80%, the films still acted as a good oxygen barrier, having an oxygen permeability of 5.5 mL·µL/(m(2)·24 h·kPa). These properties indicate that this new material has a potential for use as a barrier in complex-shaped structures and hence ultimately reduce the need for petroleum-based plastics.